If you clean the rims to examine the shape of the bottom of the grooves, that would give you some hints as to whether they were machined there by design. If you take accurate positional measurements of the grooves, relative to a reference (the OD edge of the rim, say) and relative to each other, you could further infer whether or not they were put there by design.
Having no idea about the rims, I would not wager a guess -- but if, upon inspection and measurement, the grooves do appear to be purposefully machined into the rim (i.e. matching shape and position), I think it would be safe to say that they are designed as wear indicators.